This invention relates to a drag-type drill bit for downhole drilling of a bore hole, and more particularly to a drag-type drill bit having variable pitch generally spiral shaped blades extending from the bit face to the gage area.
Fixed cutter rotary drag-type bits for earth boring were developed several decades past and have been in use for bore-hole drilling in various subterranean formations. During the development of the drag-type drill bit, the primary objective focus was high penetration rates, long drill bit useful life, and dynamic stability (xe2x80x9cbit whirlxe2x80x9d correction). Various efforts were made to address the dynamic stability issue including elongated gage surfaces and a 360 degree full contact gage, both intended to restrict lateral vibrations thereby reducing bit whirl and enabling bit steerability, that is, directional drilling. Improvement of dynamic stability of a drag-type drill bit resulted in less bore hole enlargement and in smooth trajectories for directional drilling, all obtained with less load loss, more predictable weight transfer and improved well completion.
Along with the development of improved dynamic stability it has also become common practice to employ cutting elements made of man-made polycrystalline diamond compacts or cutters projecting from the bit body of a drag-type drill bit. One polycrystalline diamond cutting structure in common use has been commonly referred to as polycrystalline diamond compact (PDC) comprising a small carbide cylindrical body with a thin layer of polycrystalline diamond bonded to one face thereof. This is a conventional PDC-type diamond drill bit cutting element capable of drilling in softer formations.
Although development of the PDC cutting element has resulted in more extensive use of drag-type drill bits there has been an undesirable increase in problems associated with heat degradation resulting from xe2x80x9cballingxe2x80x9d. Balling is defined as a buildup of formation chips or cuttings on the bit face and gage area or the bore hole bottom and is most often a problem with sticky formations, such as sticky shales or similar formations having a large percentage of clays that adhere to the cutting face of the bit. This balling condition not only hampers drilling activity, but also causes rapid heat deterioration of the cutting elements due to poor circulation and decreased cutting efficiency.
A number of efforts have been made to correct the balling condition such as a deep-bladed design, however, this resulted in considerable wear and breakage when harder formations were encountered because of the relatively small number of cutting elements. Recent computational fluid dynamics flow analysis indicates that hydraulic efficiency is seriously compromised in drag-type drill bits with high spiral blades supporting the cutting elements. The spiral blade configuration induces a specific fluid flow trajectory systematically projected on the back of the adjacent spiral blade thereby compromising cutting structure cleaning and thus an accumulation of sticky formation cuttings thereby inducing xe2x80x9cballingxe2x80x9d. Further, computational fluid dynamics has shown that because of the very high spiral angle, the fluid trajectory near the gage area is almost circumferential thereby inhibiting the desired flow of cuttings from around the bit past the supporting drill string to the surface of the bore hole. The conclusion reached from the recent computational fluid dynamics flow analysis is that the trajectories of the drilling mud from the nozzle orifices of the drill bit face result in low velocity zones and recent fluid tests correlate the results of the fluid dynamic flow analysis.
Another approach to address the balling problem was to utilize a high density of cutting elements and fluid nozzles directed to the well bore bottom. After the fluid impinges the well bore bottom a portion of the fluid flows at relatively low velocity through the flow channels between the plurality of blades. The fluid velocity in these channels is too low for providing adequate cleaning of the cutting elements when drilling in soft sticky formations to prevent balling. Other attempts to address the balling condition utilized a relatively large number of nozzles in an effort to adequately clean all the cutting elements on the bit. A reduction in velocity results when the total orifice area for the bit is increased beyond a reasonable limit and results in an increase of the probability of clogging of the nozzle orifices.
The development of a PDC drag bit with high spiral blades presents some design constraints related to the side rake angle parameter. The cutters have to be normal to the cutting trajectory, and have to be oriented in a side direction (cutter side rake angle). The combination of the spiral blade shape and the three-dimensional positioning of the cutters causes collisions to occur between some cutters. One solution for overcoming this problem used in the past was to reduce the number of cutters. This affects the cutting process efficiency.
The present invention comprises a drag-type drill bit having a bit body with a plurality of blades extending from the bit face to the gage area, the blades have a general spiral shape with a variable pitch from the bit face to the gage area. The spiral pitch either increases or decreases from the bit face to the gage area with a positive or negative orientation for blades of a drill bit rotating from right to left. Each of the plurality of blades comprises a plurality of segments having a definable spiral pitch value (positive or negative) depending on the direction of the bit rotation. Depending on the direction of the bit rotation, the plurality of segments define right spiral pitch or left spiral pitch for the plurality of blades.
Variable pitch spiral blades can be obtained by a succession of segments with different curvature radius as well as by spline curves defining essentially the same shape.
Further in accordance with the present invention there is provided a drag-type drill bit having a plurality of blades extending above the bit face to the gage area wherein the spiral pitch of each blade is constant along the length of the blade (general virtual shape) with the front and/or back faces of a blade having a variable spiral pitch (a wave-shaped face).
In accordance with the present invention there is provided a drag-type drill bit with variable pitch spiral blades having a succession of concave and/or convex segments (succession of concave segments with different pitch values, succession of convex segments with different pitch values, succession of convex-concave segments with different pitch values).
The drag-type drill bit of the present invention comprises a bit body having a threaded pin on one end for coupling to a drill string. A substantially dome-shaped cutter head is formed integral with the bit body opposite from the threaded pin. A plurality of circumferentially spaced wave-shaped blades are formed integral with and extending from the cutter head. The wave-shaped blades comprising a variable pitch pattern having a succession of curved segments, the variable pitch pattern curving from substantially the center of and around the cutter head to the gage area in the direction of rotation of the bit body. A plurality of generally cylindrical cutting elements are embedded in each of the wave-shaped blades and a plurality of nozzles are positioned around the cutter head to direct drilling fluid passing through the bit body against the wave-shaped blades to the annulus surrounding the bit body in the bore hole.
Further in accordance with the present invention there is provided a drag-type drill bit for bore hole drilling in earth formations comprising a bit body having a threaded pin end for coupling to a drill string. A substantially dome-shaped cutter head is formed integral with the bit body opposite from the threaded pin. A first plurality of circumferentially spaced wave-shaped blades are formed integral with and extend from the cutter head, the first plurality of wave-shaped blades comprising a variable pitch pattern having a succession of curved segments or spline curves, the variable pitch pattern curving from substantially the center of and around the cutter head to the gage area in the direction of rotation of the bit body or in a direction opposite to the rotation of the bit body. A second plurality of circumferentially spaced blades are formed integral with and extend from the cutter head between adjacent wave-shaped blades of the first plurality, blades of the second plurality comprising a fixed pitched pattern curving from a position displaced from the center of and around the cutter head to the gage area also in the direction of rotation of the bit body or in the opposite direction. A plurality of generally cylindrical cutting elements are embedded in each of the first plurality of wave-shaped blades and the second plurality of blades, and a plurality of nozzles are positioned around the cutter head to direct drilling fluid passing through the bit body against the first plurality of wave-shaped blades and the second plurality of blades to the annulus surrounding the bit body and the bore hole.
Technical advantages of the drill bit of the present invention includes hydraulic efficiency based on the variable pitch pattern of spiral blades extending above the bit face of the bit body to the gage area. An additional technical advantage is the utilization of the concavity of variable pitch spiral blades to function as a deflector for fluid flow from a nozzle thereby improving hydraulic flow efficiency. A further technical advantage of the present invention is a drag-type drill bit having variable pitch blades creating fluid channels near fluid nozzles and the gage area to clean cutting elements on the bit shoulder and evacuate efficiently the drilling fluid and rock chips towards the annulus between the drill string and the walls of the bore hole. An additional advantage includes cutting efficiency based on the use of cutters with side rake angles.